The legibility of displays is a major preoccupation in horology, especially for calendar type displays, which are difficult to make in formats that are easy for the user to see and decipher.
Timepiece displays are rarely made using rollers since indications taking this form require high thickness due to the diameter of the roller, comprising, for example, up to 31 indications for the days of the month, or 52 indications for the weeks of the year, and are incompatible with the specific geometry of a watch.
Also, the possible use of characters of very small size requires the use of magnifying lenses in the thickness of the watch crystal, which is highly detrimental to the aesthetic appearance of the watch, while remaining difficult to read.
Static displays using flaps or blades for miniature and other clocks, are difficult to transpose to watches since they generally rely on gravity. They are more fragile and cannot withstand shocks.
U.S. Pat. No. 3,964,428 in the name of ARAI KIYOYUKI describes an indicator with rotating flaps having two sides, at the periphery of a rotating drum, with means for guiding the flaps, held in a tangential orientation. During rotation of the drum, a mechanism allows the flaps to be overturned about their axis.